Emptor queritur sex sestertios nimium esse.

Emptor means buyer. It is nominative singular because it is the subject of the main verb queritur.

Its dictionary form is emptor, emptoris, so this is a 3rd-declension noun.

Because queritur comes from queror, which is a deponent verb.

Deponent verbs:

• have passive-looking forms
• but active meanings

So queritur is literally a passive-looking form, but it means he/she complains or here the buyer complains.

It is:

• 3rd person singular
• present tense
• indicative mood

It is an indirect statement, also called an accusative-and-infinitive construction.

In this construction:

• the subject of the subordinate clause goes into the accusative
• the verb goes into the infinitive

So here:

• sex sestertios = the subject of the indirect statement, in the accusative
• esse = the infinitive
• nimium = the predicate idea, too much

The whole phrase means that six sesterces is too much.

Because Classical Latin usually does not use a separate word like English that in this kind of sentence.

English says:

• The buyer complains that six sesterces is too much

Latin usually says:

• The buyer complains six sesterces to be too much

That sounds strange in English, but it is a normal Latin pattern.

Because in an indirect statement Latin uses the infinitive, not a finite verb.

So instead of is/are, Latin uses to be:

• esse = to be

If you turned the idea into a direct statement, you would use a finite verb instead, something like:

• Sex sestertii nimium sunt

In the given sentence, though, it has to be esse because it depends on queritur.

Because it is the subject of the infinitive inside the indirect statement.

That is one of the most important things to get used to in Latin:

• the subject of a normal finite verb is nominative
• the subject of an infinitive in indirect statement is accusative

So:

• direct statement: sex sestertii
• indirect statement: sex sestertios

To an English speaker, an accusative subject feels odd, but it is completely normal in Latin.

Because sex is an indeclinable numeral. It means six, and it stays sex in all cases.

The case is shown by the noun that goes with it:

• nominative: sex sestertii
• accusative: sex sestertios

So the ending changes on sestertii/sestertios, not on sex.

Because nimium here does not mean too many in the ordinary adjective sense. It means too much.

Latin often uses a neuter singular form in this kind of predicative expression. So nimium is functioning more like:

• too much
• excessively much

If Latin wanted simply too many sesterces, with an adjective agreeing directly with sestertios, you would expect something like nimios sestertios. But that is a slightly different idea.

So:

• nimios sestertios = too many sesterces
• sex sestertios nimium esse = that six sesterces is too much

Sestertios is the accusative plural of sestertius, a Roman coin.

Its basic forms are:

• nominative singular: sestertius
• nominative plural: sestertii
• accusative plural: sestertios

So in this sentence sex sestertios means six sesterces.

No. Latin word order is much freer than English word order.

This sentence is arranged in a very natural way:

• Emptor first as the main subject
• queritur next as the main verb
• then the content of the complaint
• esse at the end, which is very common for an infinitive

Latin could rearrange the words for emphasis, but the endings would still show the grammar. So the order helps style and emphasis more than basic comprehension.